The dead time of a nuclear imaging system is the time during which the system processes a single event (i.e., the interaction of a particle or stimulus from a radiation field with the system) and is not available to process a succeeding event. It arises because of the multitude of electronic circuits in a nuclear imaging system, each with its own dead time, and the complex interaction between such circuits. Furthermore, the count rate losses of a system are a function of the number of particles produced by the radiation field under investigation which lie outside the energy window of the single channel analyzer of the system because the interactions of such particles occupy the circuitry of the system while a decision is being reached with regard to further processing. Thus, the dead time of a nuclear imaging system depends on the nature of the system and the type of field interacting therewith.
As a consequence of the dead time phenomenon in nuclear imaging systems, the rate at which events are processed by the system is a non-linear function of the rate of incoming events. For a typical conventional gamma camera, the curve relating events processed to incoming events peaks at about 200,000 counts per second which defines the so-called foldback point of the curve. At such point, the typical camera processes only about 50% of the incoming events; while at greater counting rates, the efficiency of the camera drops below 50%. Thus, if a radiation field produces particles that interact with the camera at rates in excess of 200,000 per second, less than half of these events will be processed by the camera and appear in a map of the radiation field.
The inherent variable dead time losses in a gamma camera do not significantly degrade images of static radiation fields. For example, it is conventional to obtain an image of an organ such as the liver or thyroid by injecting a patient with a radioactive pharmaceutical that gravitates to the organ of interest and remains there for a relatively long time as compared to the time required to obtain an image of the organ using the nuclear imaging system. In this case, the intensity distribution of counts over an area is desired and the efficiency of the camera in processing events has no significant importance.
However, where physiological dynamic mechanisms are being studied, such as the rate at which blood is washed out of the brain or kidney, or the quantity of blood pumped by the heart in each cycle, the quantity/time behaviour of the incoming events presented to the camera becomes crucial because the quantitative information indicates the degree of abnormality in the function of an organ. For example, a radiopharmaceutical injected into the vascular system of a patient will reach the heart essentially as a substantially undiluted bolus. The initial contraction of the heart will draw this bolus into the heart pool which, as a result will have an activity that will produce, say, 500,000 particles per second that interact with the camera, but only for a few seconds before a portion of the pool is expelled by the contraction of the heart. The radiopharmaceutical remaining in the heart is diluted by the next cycle of the heart, reducing further the radioactivity, and thus reducing the number of events presented to the camera. After five or six contractions, the amount of radiopharmaceutical remaining in the blood pool in the heart is so reduced that coming events to the camera are reduced to, say, 50,000 particles per second.
If the above described process were imaged with a conventional gamma camera uncompensated for its dead time, its inefficiency due to the dead time phenomena would suppress the processing of events in direct proportion to the rate at which these events are presented to the camera. That is to say, the greater the rate of nuclear activity in the heart, the higher the losses in the gamma camera which, as a consequence, would yield distorted data that conceals the true situation. Any diagnosis based on such data is likely to be incorrect.
One approach to compensating for the dead time of a gamma camera in order to take into account the dependency of the efficiency of the camera on the rate of incoming events is to assume an analytical approximation of the statistical probability of an event that encounters an electronic component in the camera will be operated on by such component. As indicated previously, dead time is extremely complicated and is dependent not only on the inherent limitations of the camera itself but on the nuclear spectra which the camera is associated. As a consequence, the use of an analytical function to compensate for dead time yields high error.
It is therefore an object of the present invention to provide a new and improved method of and means for compensating for the dead time of a gamma camera which is less complex than the techniques of the prior art and more likely to produce accurate results.